2 eV band gap tuning and optical properties of AgIn5S8 Quantum Dots

Abstract

In this work, we demonstrate the colloidal bottom-up synthesis of spinel AgIn₅S₈ quantum dots (QDs) with tunable optical properties. The QD size, and consequently their band gap energy (Eg), is effectively controlled by reaction temperature and ultrasound (US) irradiation. Under combined conditions of 75 °C and US irradiation, ultrasmall QDs with an average size of 2.6 nm are obtained, exhibiting a wide band gap of 3.77 eV. In the absence of US, reactions conducted at 55 °C and 75 °C yield larger QDs (~5 nm and 31 nm, respectively), with reduced band gaps of ~3.1 eV and 2.2 eV. The elevated temperature (75 °C) suppresses sulfur-chain formation that otherwise limits growth at 55 °C, while acoustic cavitation induced by US enables narrowest size distribution. Annealing of as prepared QDs, at 200 °C for 2 h, promotes coalescence resulting in QDs with increased size of ~ 34 nm, with a bulk like band gap of 1.73 eV for QDs prepared without US. In contrast, annealing of the QDs, prepared with US, results in polycrystalline QDs with average size of ~ 21 nm. High-resolution transmission electron microscopy reveals a strong correlation between QD size, structural ordering and optical behavior. The as-prepared 2.6 nm QDs exhibit lower Urbach energy, attributed to their single-crystalline nature, unlike the less ordered QDs synthesized without US. Annealing improves structural ordering and reduces Urbach energy in QDs prepared at 75 °C, while stacking faults and grain boundaries in other QDs hinder such improvements. Photoluminescence measurements further confirm a strong relationship between QD structure, size, and emission characteristics. The synthesized AgIn₅S₈ QDs exhibit remarkable band gap tunability of up to 2 eV across the visible spectrum and sharp band-edge emission, underscoring their potential for applications in optoelectronic and biomedical devices. This work provides a robust and sustainable pathway to high-performance, non-toxic QDs, addressing a key bottleneck for their use in biocompatible and consumer electronics.